The present invention generally relates to an active antenna system for use in an aircraft, and more particularly to an active phased array antenna system for use in an aircraft surveillance system.
Today, systems exist for use in aircraft surveillance for collision avoidance and traffic alert. These conventional systems use active interrogation of Mode Select (Mode-S) and Air-Traffic Control Radar Beacon System (ATCRBS) transponders that can incorporate a passive phased array antenna. Conventional Mode-S and ATCRBS transponders transmit encoded messages containing information about the aircraft in response to interrogation signals received from ground based radar or from an aircraft with a Traffic Advisory System (TAS), or Traffic Collision Avoidance System (TCAS). When the transponder is not transmitting, it monitors for transmissions including interrogation signals.
The Minimum Operating Performance Specifications (MOPS) for the TCAS II system is described in RTCA document DO-185A, “Minimum Operational Performance Standards for Air Traffic Alert and Collision Avoidance System II (TCAS II) Airborne Equipment”, dated December 1997 and the MOPS for TCAS I and TAS are embodied in RTCA document DO197A, “Minimum Operational Performance Standards for Active Traffic Alert and Collision Avoidance System I (Active TCAS I)” both of which are incorporated herein by reference.
TAS, TCAS I and TCAS II equipment transmit interrogation signals that are received and replied to by other aircraft and used to determine the location of other aircraft relative to the originating aircraft position. Conventional TAS, TCAS I and TCAS II systems may include a 4-element interferometer antenna coupled, to a remote radio frequency (RF) transmitter/receiver. The transmitter and receiver are coupled to the antenna array by multiple low loss coaxial transmission lines. The coaxial transmission lines may be several feet in length (e.g. 30 feet long). The antenna arrays utilized by conventional TCAS systems are “passive” in that all of the power utilized to drive the antenna array elements is produced at the remote transmitter assembly. Similarly, all of the power that is used to boost the receive range of the antenna array is provided at the remote receiver assembly.
The transmitter and receiver are in turn coupled to a signal processor that controls transmission and reception of TAS and TCAS related information and that performs aircraft surveillance operations, such as traffic alert and collision avoidance operations. The transmitter is coupled to the signal processor for transmitting, among other things, interrogation signals. A control panel and display are joined to the signal processor for operating the TAS/TCAS system and for displaying TAS/TCAS information.
The TCAS system identifies the location and tracks the progress of aircraft equipped with beacon transponders. Currently, there are three versions of the surveillance systems in use; TAS, TCAS I, and TCAS II. TAS is the simplest and least expensive of the alternatives, TCAS I, is less expensive but also less capable than TCAS II. The TAS and TCAS I transmitter send signals and interrogate ATCRBS transponders. The TAS and TCAS I receivers and displays indicate approximate bearing and relative altitude of all aircraft within the selected range (e.g., about forty miles). Further, the TAS and TCAS systems use color coded dots to indicate which aircraft in the area pose a potential threat (e.g., potential intruder aircraft). The dots are referred to as a Traffic Advisory (TA). When a pilot receives a TA, the pilot then visually identifies the intruder aircraft and is allowed to deviate up to +300 feet vertically. Lateral deviation is generally not authorized. In instrument conditions, the pilot notifies air traffic control for assistance in resolving conflicts.
The TCAS II system offers all of the benefits of the TCAS I system, but also issues a Resolution Advisory (RA) to the pilot. In the RA, the intruder target is plotted and the TCAS II system determines whether the intruder aircraft is climbing, diving, or in straight and level flight. Once this is determined, the TCAS II system advises the pilot to execute an evasive maneuver that will resolve the conflict with the intruder aircraft. Preventive RAs instruct the pilot not to change altitude or heading to avoid a potential conflict. Positive RAs instruct the pilot to climb or descend at a predetermined rate of 2500 feet per minute to avoid a conflict. TCAS II is capable of interrogating Mode-C and Mode-S. In the case of both aircraft having Mode-S interrogation capability, the TCAS II systems communicate with one another and issue de-conflicted RAs.
Each of the above-described surveillance systems may utilize a passive phased antenna array that is coupled, via coaxial transmission lines, with a remote transceiver that controls transmit and receive operations. The individual antenna elements of the phased antenna array are coupled to separate transmission lines. Conventional phased antenna arrays are “passive” such that, during a transmit operation, the remote transceiver supplies transmit signals over the corresponding transmission lines at high power levels. The high power is provided from a high power source that is located at the transceiver, remote from the passive antenna array. The remote power source amplifies the transmit signals supplied to the transmission lines with sufficient power to drive the remote antenna elements with an input power of about 200 Watts (W), such as to provide a transmit range of 45 nautical miles given typical directional antenna gain.
During a receive operation, signals that are received by the antenna elements are conveyed at very low power levels without any power boosting from the passive antenna array over the transmission paths to the transceiver. Receiver systems incorporating passive antenna arrays have an inherent receive range that is dependent, among other things, upon the characteristics of the passive components provided along the receive path, noise, loss, sensitivity and the like. The receive characteristics of the components improve with cost. The sensitivity of the receive path directly impacts the receive range. Therefore, as sensitivity decreases, range similarly decreases. In certain applications, it is desirable to provide a receive range of 100 nautical miles which may not be possible with these conventional passive systems.
Passive antenna arrays have experienced limitations in the overall receiver system performance, system range and transmit power efficiency. The transmit path (e.g. transmission lines and passive components at the antenna array) forms a lossy path that attenuates the high power transmit signals during propagation from the transceiver to the passive antenna array. The power efficiency of the transmitter is limited due, in part, to transmission losses suffered while conveying high power over the coaxial transmission lines. The transmit power losses impact the system transmit range.
To maintain a desired performance, the output power of the transmitter must be relatively high to compensate for system losses. The losses may result from coaxial cable loss between the transmitter and the antenna array. Losses may occur due to internal effects in the transceiver such as high power beamsteering or high power attenuation circuits inherent in previous TAS/TCAS designs that respectively do beam forming of directional antennas, and transmit power attenuation in a process known as Whisper-Shout. Losses resulting from the coaxial cables and these other sources drive the need for high power amplifiers at the transmitter. For example, a conventional transceiver may need to generate as much as 1000 W (250 W per channel) of power at the output of the transmitter in the transceiver, while the input to the antenna array is only 200 W (50 W per channel) due to losses experienced in the transceiver and along the transmission path. An antenna with an input power of 200 W (effective radiated power or ERP is defined as the product of the transmit power times the antenna gain, typically 3 dB or twice the incident power for a directional antenna, 400W in this example) may only have an effective broadcast range of up to 45 Nautical miles.
The system also experiences limitations in the receive range and receive sensitivity due to the losses and noise produced by the coaxial transmission line and associated circuit components at the receiver and transmitter that maintain desired performance characteristics of the remotely located passive antenna array. For example, a conventional system may only be able to provide receive sensitivity for up to 90 Nautical miles. The limited receive sensitivity is due in part to cable losses, in part to noise within the receive signal path and the like.
A need exists for an improved antenna system that does not experience excessive degradation of the receive system performance, system range or transmit power losses.